Since the advent of butane blending along pipelines and at petroleum tank farms, pipeline operators and gasoline distributors have been able to blend butane into the nation's gasoline pool in a manner that optimizes the quantity of butane added to the gasoline, without violating a geographic region's volatility requirements. Methods of performing these blending operations are described, for example, in U.S. Pat. Nos. 6,679,302, 7,631,671 and 8,192,510 to Mattingly and Vanderbur. Butane is especially useful in these blending operations because of its consistent physical contribution to volatility and octane in a blended gasoline pool.
One of the problems with these methods is that the quantity of butane that can be added to a fuel stream is limited, due to the high volatility of butane. Indeed, no more than 3-5% butane is typically added to a fuel stream even in high-blending seasons. There are other sources of hydrocarbons that increase the volatility of fuels less than butane, and that conceivably could be added in greater quantities, but most of these other sources suffer from other disadvantages, such as variability in hydrocarbon content and an unpredictable effect on octane. Mixed pentanes, raw butane, and other hydrocarbons that contain n-pentane are a good example. The abundance of these hydrocarbons is increasing as new sources of energy are discovered around the world. However, these hydrocarbons cannot be readily substituted for butane due to variability in their hydrocarbon content, and uncertainty about how much impact they will have on the volatility and octane of the fuel stream. This is especially true for hydrocarbon additives that contain large amounts of n-pentane, which has a neat octane value of only 65, and whose effect on fuel octane is unknown.
Given the number and types of fuels transmitted through our nation's pipelines, and the eventual blending of many fuel streams with ethanol, further complications arise from variability within the fuel stream itself. It is well known that the quantity of aromatics in a gasoline batch can have a significant impact on the Reid vapor pressure (RVP) blending values of non-aromatic hydrocarbons, and that RVP typically increases with the aromatic content of the gasoline, a so-called “aromatic effect.” These variations make it difficult to blend hydrocarbons into fuel streams, especially fuel streams that have been blended to meet demanding certification requirements, including strict limits on volatility and octane. This is especially true when inconsistent additives such as mixed pentanes or raw butane, which vary in terms of volatility and octane, are used in the blending process.
What is needed are new methods that permit less well defined hydrocarbons such as mixed pentanes and raw butane to be blended into fuels downstream of the refinery. Blending methods originally developed for butane, that project the impact of the blending on the volatility of the blended fuel, must be adapted to permit blending of n-pentane, mixed pentanes, and raw butane, into fuels received from the refinery, without negatively affecting the volatility or octane value of the fuels received from the refinery.